Expression system for producing collagen

ABSTRACT

A DNA molecule consisting of the nucleotide sequence of SEQ ID NO: 1, which encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen. Expression systems and methods for the expression of the DNA molecule are also provided.

BACKGROUND

The invention relates to expression systems for producing collagen; more particularly, the invention relates to co-expression of collagen and prolyl 4-hydroxylase in stably transfected insect cells.

Collagens are extracellular matrix proteins that contain the repeating triplet sequence Gly-X-Y and the presence of such triplets allows three collagen polypeptide chains (α-chains) to fold into a triple-helical conformation. The Y position amino acid in the triplet sequence Gly-X-Y is frequently proline, which is often 4-hydroxylated by post-translational modification of the collagen polypeptide chain in order to stabilize the triple-helical structure of collagen. In the absence of proline hydroxylation, the essential triple helical conformation of collagen is thermally unstable at below physiological temperatures (Berg, R. A., and Prockop, D. J. (1973) Biochem Biophys Res Commun 52, 115-120; Rosenbloom, J., et al. (1973) Arch Biochem Biophys 158, 478-484). Prolyl 4-hydroxylase (EC 1.14.11.2), the key enzyme catalyzing the 4-hydroxylation of proline residues in all collagens, from vertebrates is an α₂β₂ tetramer consisting of two different types of subunits (Kivirikko, K. I., et al., (1989) Faseb J3, 1609-1617). The α subunit contains the catalytic and peptide-substrate binding domains but is inactive in the absence of the β subunit. The β subunit was found to be identical to the enzyme protein-disulfide isomerase (Koivu, J., et al. (1987) J Biol Chem 262, 6447-6449; and Pihlajaniemi, T., et al. (1987) Embo J6, 643-649). During collagen biosynthesis, the procollagen α chains are co-translationally transported into the lumen of the endoplasmic reticulum where they are hydroxylated by prolyl 4-hydroxylase. Prolyl 4-hydroxylase requires Fe²⁺, 2-oxoglutarate, O₂ and ascorbate, and an active system appears to exist in vertebrate cells for the transport of 2-oxoglutarate and ascorbate into the lumen of the endoplasmic reticulum (Kivirikko, K. I., et al. (1989) Faseb J3, 1609-1617). In vitro expression of an active recombinant prolyl 4-hydroxylase from its subunits has been successfully obtained by co-infection of insect cells Spodoptera frugiperda and Trichoplusia ni (Vuori, K., et al. (1992) Proc Natl Acad Sci USA 89, 7467-7470) with recombinant baculoviruses, or cotransfection with expression vectors in mammalian cell lines COS-1 (John, D. C., and Bulleid, N. J. (1996) Biochem J 317 (Pt 3) , 659-665) and HEK293 (Wagner, K., et al. (2000) Biochem J352 Pt 3, 907-911), in yeasts Pichia pastoris (Vuorela, A., et al. (1997) Embo J16, 6702-6712) and Saccharomyces cerevisiae (Toman, P. D., et al. (2000) J Biol Chem 275, 23303-23309). The prolyl 4-hydroxylase tetramer assembly probably requires molecular chaperones, such as immunoglobulin heavy chain binding protein, BiP (John, D. C., and Bulleid, N. J. (1996) Biochem J 317 (Pt 3), 659-665; Veijola, J., et al. (1996) Biochem J315 (Pt 2), 613-618).

Expression systems for producing recombinant collagens in the present includes E. coli, yeast, mammalian cell lines, insect cells, as well as transgenic animals and plants, however, these expression systems have their own disadvantages. For example, E. coli expression system has no post-translational modification, and yeast expression system lacks prolyl 4-hydroxylase activity, through the yield of collagens in Pichia pastoris was the highest among these expression systems. Mammalian cell line expression system has low yield and limits to specific tissue types, and insect cell expression system has low prolyl 4-hydroxylase activity. As for transgenic animals, such as silk worm or mice, or plant, such as tobacco, the collagen products were overly cross-linked. A baculovirus expression system containing genes encoding human prolyl 4-hydroxylase and collagen has been established (U.S. Pat. Nos. 5,077,214 and 5,593,859), however, the transfected insect cells will be destroyed in 72 hours, resulting in only transient recombinant protein production. In addition, it is difficult to recover or purify the recombinant collagens because of cell lysis. Moreover, collagens are secreted or membrane proteins, and the destruction of endoplasmic reticulum or Golgi body by the baculovirus limits the production of collagens. An expression system for producing biologically active collagens in high yield is, therefore, still required.

SUMMARY

The inventors established stable insect cell lines producing the α and β subunits of P4H (human prolyl 4-hydroxylase) in both Trichoplusia ni and Drosophila melanogaster S2 expression systems. Type XXI minicollagen comprising the intact C-terminal noncollagenous (NC1) and collagenous domain (COL1) was used as a model to characterize the collagen structure and chain assembly in the Drosophila system by coexpression of the three genes in a stably transformed manner. The invention is, thus, achieved.

Accordingly, an embodiment of the invention provides a recombinant insect cell. The recombinant cell includes a transfected gene encoding human prolyl 4-hydroxylase. The recombinant cell further includes a DNA molecule consisting of the nucleotide sequence of SEQ ID NO: 1. The DNA molecule encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen. The cell can be a Trichoplusia ni cell or a Drosophila melanogaster cell.

Another embodiment of the invention provides a method for producing a recombinant collagen. The method includes the steps of providing a recombinant insect cell including a transfected gene encoding human prolyl 4-hydroxylase; transfected an expression vector comprising a recombinant collagen gene into the cell; culturing the cell under conditions such that the recombinant collagen gene is expressed; and recovering the expressed recombinant collagen.

Yet another embodiment of the invention provides a method for producing a recombinant collagen. The method includes the steps of: providing a recombinant insect cell comprising a transfected gene encoding recombinant collagen, and a transfected gene encoding prolyl 4-hydroxylase; culturing the recombinant insect cell under conditions such that the recombinant collagen is expressed; and recovering the expressed recombinant collagen.

The other embodiment of the invention provides a DNA molecule consisting of the nucleotide sequence of SEQ ID NO: 1. The DNA molecule encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen. A recombinant collagen encoded the DNA molecule is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention can be more fully understood and further advantages become apparent when reference is made to the following description and the accompanying drawings in which:

FIGS. 1A and 1B illustrate Western blot analyses of the soluble protein extract of Trichoplusia ni (A) and Drosophila S2 (B) cells harboring the stably transfected P4Hα and P4Hβ expression constructs. The P4H stably transfected Drosophila S2 cells were either cultured alone (B, lanes 1 and 3) or induced with 0.5 mM of copper sulfate and kept growing for 120 h before harvesting (B, lanes 2 and 4). The cells were broken in a buffer containing 0.2% Triton X-100 and about 50 μg of soluble cell extract were used for immunoblotting.

FIGS. 2A and 2B illustrate Contribution of human prolyl 4-hydroxylase in the assembly of type XXI minicollagen in Drosophila S2 cells. FIG. 2A illustrates the predicted primary sequence of recombinant mC21 (SEQ ID NO: 2). The Drosophila expression construct pMT/BiP-mC21 contains the sequences encoding the Drosophila BiP signal sequence (shaded area), the last four amino acid residues of the NC2 domain, the entire COL1 and NC1 domains of human α1(XXI) collagen, and a C-terminal histidine tag. The predicted N-terminus after cleavage of the signal peptide is indicated by the downward pointing arrow. Prolyl residues predicted to be hydroxylated are indicated by a large dot. The FACIT collagen family characteristic two GXY imperfections and the two cysteinyl residues at the COL1/NC1 junction involved in interchain disulfide bridging are indicated in italic and boldface, respectively. FIG. 2B illustrates Western blot analysis of mC21 molecules expressed in Drosophila S2 cells. The culture media from the stably transfected Drosophila cells 120 h post-induction with Cu²⁺ was electrophoresed on a 10% SDS/Bis-Tris polyacrylamide gel with MES buffer under non-reducing (lanes 1 and 2) and reducing (lane 3) conditions and then immunoblotted with a monoclonal antibody to the C-terminal of type XXI collagen, 3E2. Lane 1, the culture media of Drosophila cells stably transfected with mC21 alone (mC21ΔP4H); Lanes 2 and 3, the culture media of Drosophila cells coexpressing P4Hα, P4Hβ and mC21 (mC21/P4H). T, interchain disulfide-bonded trimers; D, interchain disulfide-bonded dimers; M, monomers.

FIG. 3 illustrates time-course analysis of protein expression in Drosophila cells containing stably transfected P4Hα, P4Hβ, and minicollagen XXI genes. The Drosophila clone was induced with 0.5 mM of copper sulfate and protein expression level for P4Hα, P4Hβ, and mC21 were examined over a 6-day time course (lanes 2 to 7). Sodium ascorbate (80 μg/ml) was added everyday. Equal amounts of cellular lysate and aliquots of culture media were prepared at indicated time intervals and were used for the detection of P4Hα, P4Hβ, and mC21 expression by Western blotting with the indicated monoclonal antibodies shown on the left. Lane 1, Stably-transfected Drosophila S2 cells without Cu²⁺ induction. T, interchain disulfide-bonded trimers; D, interchain disulfide-bonded dimers; M, monomers.

FIG. 4 illustrates purification of minicollagen XXI from the stably transfected Drosophila S2 cells. Recombinant mC21 expressed in Drosophila S2 cells alone (mC21ΔP4H) and coexpressed in the P4H stably transfected Drosophila S2 cells (mC21/P4H) were purified from culture media by column chromatographies. The samples were electrophoresed on a 10% SDS/Bis-Tris polyacrylamide gel with MES buffer under reducing conditions (lanes 1 and 3) and non-reducing conditions (lanes 2 and 4). The gel was stained with Simple Blue Safestain reagent. T, interchain disulfide-bonded trimers; D, interchain disulfide-bonded dimers; M, monomers.

FIGS. 5A and 5B illustrate MALDI-TOF mass spectrometric analysis of the tryptic fragments (residues 43-58 and 19-34 of SEQ ID NO: 2) of minicollagen XXI expressed in Drosophila S2 cells. Recombinant mC21, with and without coexpression with P4H in the Drosophila S2 cells, were electrophoresed on a 10% SDS/Bis-Tris polyacrylamide gel with MES buffer under reducing conditions. Protein bands were visualized by staining with Simple Blue Safestain reagent, followed by in-gel digestion with trypsin and MALDI-TOF mass spectrometric analysis. Two tryptic peptides derived from mC21 expressed in Drosophila S2 cells alone, mC21ΔP4H (A); and coexpressed in the P4H stably transfected Drosophila S2 cells, mC21/P4H (B) were chosen for study. Prolyl residues in the Y position of an XYG triplet sequence and thus predicted to be hydroxylated are indicated by a dot.

FIG. 6 illustrates characterization of the triple-helical structure of minicollagen XXI. Purified recombinant mC21, expressed in Drosophila S2 cells alone (mC21ΔP4H) and coexpressed in the P4H stably transfected Drosophila S2 cells (mC21/P4H), were incubated for 15 min at 22° C. (lanes 2 and 5) and 37° C. (lanes 3 and 6) and then treated with pepsin for 2 h at 22° C. Pepsin digested minicollagens were electrophoresed on a 10% SDS/Bis-Tris polyacrylamide gel with MES buffer under non-reducing conditions, and proteins were visualized by Coomassie staining. Lanes 1 and 4, undigested recombinant minicollagens XXI, mC21ΔP4H and mC21/P4H, respectively. Pepsin is indicated by the arrow. (αCOL1) ₃, trimeric disulfide-bonded COL1 domain; (αCOL1), isolated chains dissociated from trimeric non-disulfide-bonded COL1 domain.

FIGS. 7A and 7B illustrate separation of minicollagen XXI species by gel filtration. 7A. Purified recombinant mC21, coexpressed in the P4H stably transfected Drosophila S2 cells, was separated by a Superdex 200 (HR 10/30) gel filtration column equilibrated with 0.1 M NaCl, 50 mM sodium acetate (pH 6.0), without prior denaturation (a), or after heating for 15 min in the presence of 5 mM DTT (dithiothreitol) (b). Flow rate, 0.4 ml/min. 7B. The different peak fractions (numbered 1 to 3 in 7A) were analyzed by SDS-PAGE under non-reducing conditions. Gel was visualized by silver nitrate staining. T, interchain disulfide-bonded trimers; D, interchain disulfide-bonded dimers; M, monomers.

DETAILED DESCRIPTION

A recombinant collagen and a DNA molecule corresponding thereto, a recombinant insect cell including a transfected gene encoding a human prolyl hydroxylase, and a method for producing the recombinant collagen using the recombinant insect cell are provided.

Twenty-seven distinct types of homo- and heterotrimeric molecules, encoded by more than 42 genes, have been identified in vertebrates. These proteins exhibit considerable diversity in size, sequences, tissue distribution, molecular composition and each plays a different structural role in connective tissue. Triple-helical assembly of fibrillar collagens (collagen types I, II, III, V, and XI) is initiated by association of their conserved large C-terminal non-triple-helical domains (around 250 amine acids), called the C-propeptides, and then folded into a triple-helix by propagation of the three α chains from the C to N termini (Prockop, D. J., and Kivirikko, K. I. (1995) Annu Rev Biochem 64, 403-434). The FACIT (fibril-associated collagen with interrupted triple-helices) collagen family, including types IX, XII, XIV, XVI, XIX, XX, XXI and XXII, is a group of nonfibrillar collagens and some of them have been shown to connect to collagen fibrils and interact with other matrix components or cells (Fukai, N., et al. (1994) Methods in Enzymology 245, 3-28; Holden, P., Meadows, R. S., et al. (2000) J Biol Chem; Keene, D. R., Lunstrum, G. P., et al. (1991) J Cell Bio 113, 971-978; Montserret, R., et al. (1999) Biochemistry 38, 6479-6488; Pihlajamaa, T., et al. (2004) J Biol Chem 279, 24265-24273). The FACIT collagens are devoid of large C-propeptides, which are replaced by significantly shorter C-terminal non triple-helical domains (NC1). The NC1 domains of the FACITs do not share any sequence similarity. In contrast, FACITs display remarkable similarities in their NC1 adjacent triple-helical domains (COL1) and the junction of the COLl and NC1 domains, including two cysteines separated by four amino acids are responsible for interchain disulfide bonding. Previous studies on the assembly of recombinant minicollagen XII using the baculovirus expression system show that folding of the triple-helical COL1domain precedes the formation of the disulfide bonds (Mazzorana, M., et al. (2001) J Biol Chem 276, 27989-2998).

The αl(XXI) collagen (α1-chain of type XXI collagen) encodes a protein of 957 amino acid residues and possesses a putative 22-residue signal peptide and 2 COL domains interspersed with 3 NC regions. The inventors' previous study indicated that type XXI collagen is an extracellular matrix component of the blood vessel walls and the expression is developmentally regulated (Chou, M. Y., and Li, H. C. (2002) Genomics 79, 395-401). So far, type XXI collagen has mainly been characterized at the cDNA and genomic levels (Chou, M. Y., and Li, H. C. (2002) Genomics 79, 395-401; Fitzgerald, J., and Bateman, J. F. (2001) FEBS Lett 505, 275-280; Tuckwell, D. (2002) Matrix Biol 21, 63-66), while studies at the protein level have been hampered by its low level of expression and the lack of suitable antibodies. Expression of the α1(XXI) collagen chain recombinantly may provide an alternative approach to study the structure and function of type XXI collagen.

Drosophila Schneider 2 (S2) cells derived from Drosophila melanogaster have been developed as a plasmid-based based insect cell system (Schneider, I. (1972) J Embryol Exp Morphol 27, 353-365). In the plasmid-based non-lytic expression system, rather than using recombinant baculovirus, high copy numbers of recombinant plasmid vectors have been shown to be inserted into the host cell genome, with the advantage that foreign proteins are expressed continuously and stably without destroying cells (Vanden Broeck, J., et al. (1995) J Neurochem 64, 2387-2395; and Johansen, H., et al. (1989) Genes Dev 3, 882-889).

The inventor established stably transfected insect cell lines containing cDNAs encoding the α and β subunits of human prolyl 4-hydroxylase in both Trichoplusia ni and Drosophila melanogaster S2 cells. The involvement of prolyl 4-hydroxylase in the assembly of the three alpha chains to form trimeric type XXI minicollagen, which comprises the intact C-terminal noncollagenous (NC1) and collagenous domain (COL1), in the Drosophila system were further characterized. When minicollagen XXI was stably expressed in Drosophila S2 cells alone, negligible amounts of interchain disulfide-bonded trimers were detected in the culture media. However, minicollagen XXI was secreted as disulfide-bonded homotrimers by coexpression with prolyl 4-hydroxylase in the stably transfected Drosophila S2 cells. Minicollagen XXI coexpressed with prolyl 4-hydroxylase contained sufficient amounts of hydroxyproline to form thermally stable pepsin-resistant triple helices consisting of both interchain and non-interchain disulfide-bonded trimers. These results demonstrate that a sufficient amount of active prolyl 4-hydroxylase is required for the assembly of type XXI collagen triple helices in Drosophila cells and the trimeric assembly is governed by the C-terminal collagenous domain.

Accordingly, one embodiment of the invention provides a recombinant insect cell including a transfected gene encoding prolyl 4-hydroxylase. The transfected gene includes α subunit and/or β subunit of prolyl 4-hydroxylase. The α subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 3, and the β subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 5. The recombinant insect cell further includes a transfected gene encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen, including the nucleotide sequence of SEQ ID NO: 1.

In the embodiment of the recombinant insect cell of the invention, the cell can be a Trichoplusia ni cell or a Drosophila melanogaster cell, preferably, Drosophila melanogaster cell.

Another embodiment of the invention provides a method for producing a recombinant collagen. The method includes the steps of: providing a recombinant insect cell; transfecting an expression vector comprising a recombinant collagen gene into the cell; culturing the cell under conditions such that the recombinant collagen gene is expressed; and recovering the expressed recombinant collagen.

In the embodiment of the method for producing a recombinant collagen, the recombinant insect cell including a transfected gene encoding human prolyl 4-hydroxylase. Specifically, the transfected gene includes α subunit and/or β subunit of prolyl 4-hydroxylase. The β subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 3, and the β subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 5. The recombinant cell can be a Trichoplusia ni cell or a Drosophila melanogaster cell.

In another embodiment of the method for producing a recombinant collagen, the recombinant collagen gene encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen, including the nucleotide sequence of SEQ ID NO: 1. The expressed recombinant collagen is secreted.

Yet another embodiment of the invention provides a method for producing a recombinant collagen. The method includes the steps of: providing a recombinant insect cell comprising a transfected gene encoding recombinant collagen, and a transfected gene encoding prolyl 4-hydroxylase; culturing the recombinant insect cell under conditions such that the recombinant collagen is expressed; and recovering the expressed recombinant collagen.

In one embodiment of the method for producing a recombinant collagen, the recombinant collagen gene encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen, including the nucleotide sequence of SEQ ID NO: 1. The expressed recombinant collagen is secreted.

In the other embodiment of the method for producing a recombinant collagen, the transfected gene encoding prolyl 4-hydroxylase includes α subunit and/or β subunit of prolyl 4-hydroxylase. The α subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 3, and the β subunit of prolyl 4-hydroxylase includes the nucleotide sequence of SEQ ID NO: 5.

In another embodiment of the method for producing a recombinant collagen, the insect cell can be a Trichoplusia ni cell or a Drosophila melanogaster cell, preferably a Drosophila melanogaster cell.

The other embodiment of the invention provides a DNA molecule consisting of the nucleotide sequence of SEQ ID NO: 1. The DNA molecule encodes a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen. A recombinant collagen encoded by the DNA molecule is also provided.

Co-expression of collagen genes and cDNAs encoding the two subunits of prolyl 4-bydroxylase in insect lepidopteran cells, including Spodoptera frugiperda and Trichoplusia ni using the baculovirus infection system has lead to the synthesis of completely hydroxylated, thermally stable collagens (Lamberg, A., et al. (1996) J Biol Chem 271, 11988-11995; Hagg, P. M., et al. (1997) Am J Pathol 150, 2075-2086; Myllyharju, J., et al. (1997) J Biol Chem 272, 21824-21830; Nokelainen, M., et al. (1998) Matrix Biol 16, 329-338; Pihlajamaa, T., et al. (1999) J Biol Chem 274, 22464-22468). However, a disadvantage of the baculovirus system is that cells are ultimately destroyed by the infected viruses, resulting in only transient recombinant protein production. In the invention, non-lytic insect expression systems both in Trichoplusia ni and Drosophila S2 cells were established to facilitate the downstream process of recombinant collagen production. Since prolyl 4-hydroxylase is the key enzyme of collagen biosynthesis, a high expression level of functional prolyl 4-hydroxylase in these stably transfected insect cell systems would ensure a success in trimeric assembly of collagen chains afterward. The results of the examples showed that the total activity of prolyl 4-hydroxylase in the P4H stably transfected Trichoplusia ni cells increased only 2-fold as compared with the endogenous enzyme in the non-transfected cell control. One possible explanation for the low expression level of P4H could be that the expression of P4H under the control of OpIE2 promoter, an immediate-early gene of Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus, is significant lower than that of polyhedrin or P10 promoters which were used in the baculovirus infection system. In contrast, the protein expression levels of both subunits of P4Hα and P4Hβ, as well as the total activity of prolyl 4-hydroxylase, coexpressed in the Drosophila S2 inducible system were 3˜4-fold higher than that in the Trichoplusia ni system, probably due to high copy numbers of recombinant plasmid vectors that were inserted into the genome of Drosophila S2 cells. The 2-oxoglutarate enzymatic assays confirmed that the stably transfected Drosophila S2 system can produce functional recombinant P4H. The subsequent results in the trimeric assembly of mC21 further proved that the P4H was functionally active for prolyl hydroxylation of the collagen chains and formation of stable triple-helical structure. These data suggest that the P4α and P4β are capable of assembling into an active α₂β₂ tetramer in the Drosophila expression system.

It is the first time to produce recombinant collagen using a three-gene expression system in the Drosophila melanogaster cells. In addition, the results of the examples provide the first biochemical evidence for triple helix formation of a recombinant human type XXI minicollagen molecule and its capability to form disulfide-linked polymers only in the presence of enough active prolyl 4-hydroxylase in Drosophila S2 cells. Based on the results from mass spectrometric analysis, there were still considerable amounts of under-hydroxylated proline residues present in the mC21 that had been coexpressed with P4H. Since Drosophila S2 cells can not survive below a cell density of 2×10⁵ cells/ml (Echalier, G. (1997) Drosophila cells in culture, Chapters 2 and 3, Academic Press, New York), it is difficult to select a single stable clone in which all three gene expression constructs of mC21, P4Hα and P4Hβ co-exist. Thus, the established stably transfected polyclonal cell population may contain cells possessing only mC21 construct, along with an antibiotic selection vector. It is, therefore, that the purified mC21 from the P4H coexpressed stable Drosophila S2 cells may contain under-hydroxylated collagen molecules. The presence of pepsin-resistant, non-interchain disulfide-bonded mC21 triple helices indicated that the trimeric assembly of type XXI collagen may be initiated from the COLl domain and disulfide bridging of the two-cysteine residues in the junction of COL1/NC1 domains was not a prerequisite for initiating the triple-helical structure formation. This is in agreement with the previous studies on the assembly of recombinant minicollagen XII using the baculovirus expression system showing that folding of the triple-helical COL1 domain precedes the formation of the disulfide bonds (Mazzorana, M., et al. (2001) J Biol Chem 276, 27989-27998). The data of the examples showed that the formation of interchain disulfide-bonded minicollagen XXI trimer is dependent on hydroxyproline content of collagen chains, suggesting that the triple-helical assembly governs fine formation of the interchain disulfide bridges.

The inventors have proved that it is possible to produce active recombinant human prolyl 4-hydroxylase and collagen molecules with stable triple helices in the Drosophila S2 cell expression system. Furthermore, this non-lytic insect cell system would seem to provide an easy way for studying the mechanisms involved in the assembly of collagen triple helix, especially for those collagens that are expressed in cells and tissues in the amount too low to be characterized at the protein level. It was found shown that both P4H and mC21 can be produced in the Drosophila S2 cell line constitutively, indicating that the Drosophila S2 cell expression system can be used for the production of various recombinant collagen types for numerous scientific and medical purposes.

Practical examples are described herein.

EXAMPLES Example 1 Production of Monoclonal Antibody of α1(XXI) Collagen

For the production of C-terminal monoclonal antibody of α1(XXI) collagen, a synthetic peptide corresponding to residues 936-957 (CDPSLCFSVIARRDPFRKGPNY) (residues 128-149 of SEQ ID NO: 2) in the NCI domain of human type XXI collagen was synthesized (AnaSpec, Inc). Two milligrams of purified peptide were conjugated to keyhole limpet hemocyanin (Pierce). The coupled peptide solution was emulsified in ImmunEasy (Qiagen) and injected intradermally into two mice for monoclonal antibodies production following standard procedures (Harlow, E., and Lane, D. (1988) Antibodies: a laboratory manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). The hybridoma 3E2 was selected for this study and the total IgG was purified from media on a protein G column according to the standard protocol.

Example 2 Construction of Recombinant Plasmids

The open reading frames coding for the α and β subunits of human prolyl 4-hydroxylase genes were amplified by RT-PCR from human aortic smooth muscle cells (Clonetics). For the expression of prolyl 4-hydroxylase in the non-lytic insect cell expression system, both the InsectSelect™ and Drosophila inducible expression (DES®) systems (Invitrogen) were used. In the InsectSelect™ system, the cDNAs coding for the α and β subunits of human prolyl 4-hydroxylase were cloned at the KpnI-XhoI, and the KpnI-Xbal sites of the expression vector pIZ/V5-His, and were named pIZ/P4α and pIZ/P4Hβ, respectively. Both the α and β subunits of human prolyl 4-hydroxylase gene expressions were under the control of OpIE2 promoter derived from the baculovirus Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus. The expression cassette of the α subunit of human prolyl 4-hydroxylase gene in pIZ/P4Hα construct was PCR amplified with primers ATCGATTATCATGTCTGGA (SEQ ID NO: 7) and CTTTGAGTGAGCATCGATC (SEQ ID NO: 8) and then was subcloned into the pIZ/P4Hβ at Cla I site. The resulting construct was named pIZ/P4H. In the Drosophila inducible expression system, the cDNAs coding for the α and β subunits of human prolyl 4-hydroxylase were cloned at the NcoI-XhoI, and the PstI-XbaI sites of the expression vector pMT/V5-HisA, and were named pMT/P4Hα and pMT/P4Hβ, respectively. Both the α and β subunits of human prolyl 4-hydroxylase gene expressions were under the control of metallothionein promoter, which is inducible by addition of copper or cadmium ions.

DNA constructs for the expression of mC21 in Drosophila S2 cells were obtained as follows. DNA corresponding to nucleotides 2484-2874 of the COL21A1 open reading frame was cloned by PCR amplification with primers 5′-TTAGATCTATTCCTGGGCCACCTGGTCCGATAG-3′ (SEQ ID NO: 9) and 5′-AATCTAGACTAATAGTTTGGTCCTTTTCTG-3′ (SEQ ID NO: 10). The PCR product was digested with BglII and XbaI, followed by cloning into the expression vector pMT/BiP-V5-HisA (Invitrogen) at the same sites. The resulting construct was named pMT/BiP-mC21.

Example 3 Establishment of Stably Transfected Insect Cell Lines Expression P4H

Expression of P4H in non-lytic insect cell expression systems both in Trichoplusia ni and Drosophila S2 cells were described below.

(1) In Trichoplusia ni Cells

In the InsectSelect™ System, Trichoplusia ni cells (High Five, Invitrogen) were transfected with pIZ/P4H using Superfect transfection reagent (Qiagan). After transfection, zeocine-resistant cells were selected at a final concentration of 400 μg/ml for 4 weeks. Ten well-isolated colonies from the antibiotic-resistant clones were picked and the protein expression levels of both P4Hα and P4Hβ from the cell lysates of each stable clones were examined by Western blot analysis with monoclonal antibodies against P4Hα and P4Hα (purchased from ICN, Ins.), respectively. Western blot analysis was as described below. SDS-PAGE was carried out using a 10% NUPAGE bis-Tris polyacrylamide gel with morpholineethanesulfonic acid (MES) buffer (Invitrogen) and proteins were stained with Simple Blue Safestain reagent (Invitrogen). After SDS-PAGE, proteins were transferred to a nitrocellulose membrane. The membrane was blocked with 5% nonfat milk in PBS containing 0.1% Tween-20 and probed with antibodies. The bound antibodies were detected with peroxidase-conjugated secondary antibodies and visualized by the SuperSignal detection reagent (Pierce). The X-ray films were scanned using a densitometer for quantitation. One of the clones showing the highest expression level of P4Hα was chosen for further studies.

Comparison of the P4Hα and P4Hβ expression levels in the Trichoplusia ni cell lysates showed that the protein expression level of P4Hα is approximately 5 times less than P4Hβ (FIG. 1A).

(2) In Drosophila S2 Cells

For the expression of P4H in the Drosophila inducible expression system, Drosophila S2 cells were co-transfected with pMT/P4Hα, pMT/P4Hβ, and pCoHygro at a ratio of 10:10:1 using Effectene transfection reagent (Qiagen). After transfection, hygromycin B-resistant cells were selected at a final concentration of 300 μg/ml for 4 weeks. The expression of both P4Hα and P4Hβ is driven by the metallothionein promoter which is inducible by addition of copper ions. After addition of 0.5 mM of copper sulfate into the cultured cells and cells were kept growing for 120 h, both P4Hα and P4Hβ protein expression levels were increased dramatically as compared to the non-induced controls (FIG. 1B).

To examine whether the recombinant P4H produced in the above non-lytic insect systems are functionally active, the enzymatic activities of P4H expressed in the Trichoplusia ni and Drosophila S2 cells were assayed by using a method based on the decarboxylation of 2-oxo[l-¹⁴C]glutarate (Kivirikko, K. I., and Myllyla, R. (1982) Methods Enzymol. 82,245-304). The results were shown in TABLE I.

TABLE I Assays for recombinant prolyl 4-hydroxylase activity in Trichoplusia ni and Drosophila S2 cells Prolyl 4- Cell lysate hydroxylase activity^(a) 100 μg^(b) Dpm Trichoplusia ni cells Non-transfected control  55 ± 15 P4H-transfected 101 ± 24 Drosophila S2 cells Non-transfected control 172 ± 22 P4H-transfected, uninduced 271 ± 23 P4H-transfected, Cu²⁺ induced 356 ± 37 ^(a)The assay is based on the hydroxylation-coupled decarboxylation of 2-oxo[1-¹⁴C]glutarate with a synthetic peptide (GPP)₁₀ as substrate. ^(b)Cells were broken in a buffer containing 0.2% Triton X-100 and about 100 μg of soluble cell extract were used in each assay. P4H-transfected Drosophila cells were harvested 120 h post-induction.

As shown in TABLE I, Trichoplusia ni cells alone contained trace amount of endogenous prolyl 4-hydroxylase activity as reported previously (Lamberg, A., et al. (1996) J Biol Chem 271, 11988-11995). The P4H activity in the stably transfected cell line increased less than 2-fold as compared with the endogenous enzyme in the non-transfected cell control. Interestingly, with the same amounts of soluble cellular proteins, the endogenous prolyl 4-hydroxylase activity in the Drosophila S2 cells is 1.7-fold higher than the P4H transfected Trichoplusia ni stable clone. The P4H activity in both non-induced and Cu²⁺-induced Drosophila S2 cell line that had been stably transfected with P4H increased around 1.6- and 2-fold than the non-transfected control, respectively. The results indicated that the Drosophila S2 system is better than the Trichoplusia ni system for expressing recombinant P4H in a non-lytic mariner. Therefore, the Drosophila S2 system was chosen for the subsequent studies of minicollagen XXI expression.

Example 4 Recombinant Expression of MC21

In the beginning, it is our attempt to express full-length α1(XXI) collagen in both Trichoplusia ni and Drosophila S2 cell systems harboring the above stably-transformed P4H genes. Unfortunately, Western blot analysis of the recombinant collagen products in both systems using antibodies against α1(XXI) collagen revealed that the full-length α1(XXI) collagen molecule was degraded into several distinct fragments (data not shown). Previous studies in the trimeric assembly of chicken minicollagen XII demonstrated that the triple helical structure formation is initiated at the C-terminal collagenous domain (Mazzorana, M., et al. (2001) J Biol Chem 276, 27989-27998). In order to see whether the C-terminal collagenous domain in α1(XXI) collagen molecule is capable of initiating trimeric assembly of functional triple helical structure, mC21 comprising the intact C-terminal noncollagenous (NC1) and collagenous domain (COL1) was expressed in Drosophila S2 cells.

FIG. 2A illustrates the predicted primary sequence of recombinant mC21. The Drosophila expression construct pMT/BiP-mC21 contains the sequences encoding the Drosophila BiP signal sequence (shaded area) in the N-terminal of mC21 for secretion purpose, the last four amino acid residues of the NC2 domain, the entire COL1 and NC1 domains of human α1(XXI) collagen, and a C-terminal histidine tag. The predicted N-terminus after cleavage of the signal peptide is indicated by the downward pointing arrow. Prolyl residues in the Y position of an XYG triplet sequence and thus predicted to be hydroxylated (Kivirikko, K. I., et al. (1992) In Post-Translational Modifications of Proteins (Harding, J. J., and Crabbe, M. J. C., Eds.) CRC Press, Boca Raton, Fla., 1-51) are indicated by a large dot. The FACIT collagen family characteristic two GXY imperfections and the two cysteinyl residues at the COL1/NC1 junction involved in interchain disulfide bridging are indicated in italic and boldface, respectively.

For the expression of mC21, pMT/BiP-mC21 was co-transfected with pCoBlast vector into Drosophila S2 cells or the stable clone that bearing functional P4H. In the Drosophila S2 system, because S2 cells do not survive below a cell density of 2×10⁵ cells/ml, it was not a feasible approach to select a single colony from a plate for expansion. All colonies from one plate were pooled as a stable cell line after blastcidine selection for three weeks with a final concentration of 30 μg/ml. Both cell lysate and culture media were analyzed by Western blotting with monoclonal antibody 3E2, which recognizes the C-terminal NC1 domain of α1(XXI) collagen. The results were shown in FIG. 2B. The culture media from the stably transfected Drosophila cells 120 h post-induction with Cu²⁺ was electrophoresed on a 10% SDS/Bis-Tris polyacrylamide gel with MES buffer under non-reducing (lanes 1 and 2) and reducing (lane 3) conditions and then immunoblotted with a monoclonal antibody to the C-terminal of type XXI collagen, 3E2.

It was found that mC21 was expressed exclusively in the culture media as a soluble form, indicating that the Drosophila BiP signal sequence directed the protein secretion successfully. When mC21 was stably expressed in Drosophila alone, two major bands corresponding to the mC21 monomers and interchain disulfide-bonded dimers with apparent molecular masses of 20 and 40 kD, respectively, were detected (FIG. 2B, lane 1). However, mC21 was able to assemble as interchain disulfide-bonded homotrimers (60 kD) in the P4H stably transfected Drosophila cell line (lane 2). Reduction of mC21 produced in the P4H stably transfected Drosophila cells showed that the dimers and trimers obtained in these conditions were indeed reducible (lane 3). These results indicated that a sufficient amount of active recombinant prolyl 4-hydroxylase is required for the assembly of disulfide-bonded trimeric mC21 molecules in the Drosophila S2 cell expression system.

Example 5 Constitutive Expression of Three Human Genes in Stably Transfected Drosophila S2 Cells

To examine whether the co-transfected Drosophila clone containing three genes of P4Hα, P4Hβ, and mC21 can produce recombinant proteins continuously, we carried out a 6-day time course study by Western blot analysis of each protein with the corresponding antibodies.

In the beginning, stably transfected mC21 and/or P4H genes were expressed in Drosophila S2 cells grown in serum free media (Hyclone) in shaker flasks. The cells were seeded at a density of 4×10⁶ cells/ml and maintained at 1×10⁷ cells/ml. Inducible expression of recombinant proteins was performed by addition of 0.5 mM of copper sulfate into the media and grown over different time course. Sodium ascorbate (80 μg/ml) was added to the culture media daily for those clones containing mC21 cDNA constructs. Western blotting analysis results were shown in FIG. 3. Lane 1 indicates non-induction control. The results showed that an increasing amounts of P4Hα and P4Hβ were detected from the cell lysates over 5 days of time span. The P4Hβ produced in the Drosophila stable clone reached a maximal amount on day 2 post-induction, whereas the P4Hα took 5 days to reach a full amount expression. The expression intensity of the three forms of mC21 was highest at day 5 post-induction. The interchain-disulfide bonded trimeric form of mC21 started to appear 48 h post-induction, and continued to increase until a maximum level was reached 5 days post-induction. Notably, the expression levels of P4Hα and mC21 trimers over the time course are in proportion, indicating that a significant amount of catalytic P4Hα in the Drosophila cells is required for the assembly of interchain disulfide-bonded trimer of type XXI collagen. Approximately 1 month following induction, the amounts of P4Hα, P4Hβ and mC21 produced by the stable clone at various passage levels were compared again. It was found that the expression intensities of the three genes were about the same at each passage, indicating that the Drosophila stable clone produced the three recombinant proteins steadily (data not shown).

Example 6 Characterization of Purified Recombinant Minicollagen XXI

Recombinant mC21, with and without coexpression with P4H in the Drosophila S2 cells, were purified from culture media by column chromatographies as described below.

Stably transfected Drosophila S2 cells were induced in the presence of 0.5 mM of copper sulfate for mC21 and P4H expression and cells were grown in conditioned media for 120 h at 27° C. Around 300 ml of filtered culture media containing C-terminal His-tagged minicollagen were applied to a non-charged His-Bind Fractogel column (1.5×8 cm, Novagen) equilibrated with 20 mM sodium acetate buffer, pH 6.0 at a flow rate of 60 ml/h. After washing with the same buffer, the minicollagen was eluted with 0.1 M of imidazole in the same buffer. The UV absorbance was monitored at 280 nm and peak fractions containing minicollagen were pooled and dialyzed against 20 mM sodium acetate buffer, pH 6.0 at 4° C. overnight. The dialysate was applied onto a HighTrap sulfopropyl column (1-ml in bed volume) equilibrated with 20 mM sodium acetate buffer, pH 6.0 at a flow rate of 60 ml/h. The column was first washed with 50mM of NaCl and then the bound minicollagen was eluted with 0.25 M of NaCl in the same buffer. The peak fractions were pooled and applied onto a ZnSO₄-charged chelating Sepharose HighTrap column (1-ml in bed volume) equilibrated with 50 mM sodium acetate buffer containing 0.1 M NaCl, pH 6.0 at a flow rate of 60 ml/h. The column was first washed with 25 mM of imidazole and then the bound minicollagen was eluted with 0.25 M of imidazole in the same buffer. The final preparation was dialyzed against 50 mM of sodium acetate, pH 6.0. The expression level of mC21, with and without coexpression with P4H in the Drosophila S2 cells after cooper sulfate induction for 5 days, were ˜3 mg/1, as estimated from the amount of purified mC21 obtained and the recovery of the purification (data not shown) . Protein concentration was determined by the Lowry assay using bovine serum albumin as the standard.

Samples of purified recombinant mC21 were electrophoresed on 10% SDS/Bis-Tris polyacrylamide gels with MES buffer under reducing conditions (lanes 1 and 3) and no reducing conditions (lanes 2 and 4). The gel was stained with Simple Blue Safestain reagent. The results were shown in FIG. 4. Three major bands corresponding to the interchain disulfide-bonded trimeric, dimeric and non-disulfide-bonded monomeric molecules of mC21 (FIG. 4, lanes 2 and 4) migrated as doublets in SDS-PAGE under non-reducing conditions as previously observed in the Western blot in FIG. 2B. Individual protein bands from each doublets from tri-, di-, and mono-molecules were excised from SDS-polyacrylamide gels and subjected to tryptic digestion using sequencing grade trypsin (Promega) with an established procedure. Recovered peptides were analyzed by MALDI-TOF (Matrix-Assisted Laser Desorption Ionization Time-of-Flight) analysis (ABI 4700 Proteomics analyzer) for peptide mapping. No differences were detected in the molecular masses of mC21 peptide fragments between each protein doublet, indicating that the doublet may result from the retention of some secondary structure even in the presence of SDS, or as a non-interchain disulfide-bonded species (data not shown). These mC21 species were reduced into monomer doublet completely under reducing conditions (FIG. 4, lanes 1 and 3).

To directly demonstrate the presence of hydroxyprolines in the recombinant mC21, as processed by the prolyl 4-hydroxylase in the stably transfected Drosophila S2 cells, amino acid composition and mass spectrometric analyses were performed on a purified sample. For amino acid analysis, purified recombinant mC21 was dialyzed against 50 mM acetic acid, hydrolyzed in 6 N of HCl at 110° C. for 24 h and subjected to amino acid analysis in a Beckman system 6300 amino acid analyzer. The results were shown in TABLE II.

TABLE II Amino acid analysis of the purified recombinant human minicollagen XXI without and with coexpression with human prolyl 4-hydroxylase in Drosophila S2 cells Recombinant human minicollagen XXI non- P4H Calculated^(a) P4H transfected co-transfected^(b) (from cDNA) Amino acid Residues^(c) Asx 9.4 ± 1.9 10.6 ± 0.2 10 Glx 14.4 ± 4.3  11.6 ± 0.4  9 Hyp 2.4 ± 0.1 5.5 ± 0.2 — Ser 8.5 ± 1.8 9.9 ± 0.2 10 Gly 29.8 ± 2.9  27.1 ± 0.5  37 His 10.0 ± 1.2  9.5 ± 0.5 7 Arg 8.5 ± 0.5 8.9 ± 0.2 9 Thr 1.6 ± 0.4 2.0 ± 0.2 0 Ala 3.0 ± 0.5 3.5 ± 0.3 1 Pro 23.2 ± 1.3  20.3 ± 2.2  31 Tyr 2.9 ± 0.3 3.0 ± 0.3 2 Val 5.6 ± 0.3 5.3 ± 0.1 5 Met 0.5 ± 0.3 0.4 ± 0.1 0 Cys 1.4 ± 0.2 1.6 ± 0.6 2 Ile 5.0 ± 0.6 5.3 ± 0.1 6 Leu 7.8 ± 1.9 8.9 ± 0.2 8 Phe 4.7 ± 0.6 4.1 ± 0.2 3 Lys 7.4 ± 1.2 8.3 ± 0.1 6 Hyp/Pro = 0.10 0.27 % Hyp/ 9 21 (Hyp + Pro) = ^(a)The calculated amino acid residues are based on the deduced amino acid sequence of mC21 after removal of the Drosophila BiP signal sequence as shown in FIG. 2A. ^(b)Purified mC21 was derived from the cell media 120 h post-induction of the cells with cooper sulfate. ^(c)The values are given as mean ± S.D., n = 2.

As shown in TABLE II, the amino acid compositions in the purified mC21, with and without coexpression with P4H in the Drosophila S2 cells, were in agreement with the composition expected based on its cDNA deduced amino acid sequence. The degree of hydroxylation (the ratio of hydroxyproline residues to total proline residues plus hydroxyproline residues) in mC21, without and with coexpression with P4H, was 9 and 21%, respectively. The theoretical value for the fully hydroxylated proline residues is 17 (FIG. 2A) and thus the degree of hydroxylation is 54.8%. The results may indicate that the proline residues in mC21 molecules coexpressed by the P4H stably transfected Drosophila S2 cells were not fully hydroxylated, even though the hydroxyproline content of type XXI collagen from nature source is unknown. To further examine the extent of proline hydroxylation in the mC21 polypeptide chain, the purified mC21 was subjected to trypsin digestion, followed by MALDI-TOF mass spectrometric analysis. Two tryptic peptides derived from mC21 expressed in Drosophila S2 cells alone (FIG. 5A); or coexpressed in the P4H stably transfected Drosophila S2 cells (FIG. 5B) were chosen for comparison. The peptide fragments, with and without coexpression with P4H in Drosophila S2 cells, still contain substantial fraction of unhydroxylated molecules with molecular masses of 1531.86 and 1583.86. Mono- and bi-hydroxylated peptides were also found in each of the tryptic fragments based on the additional molecular mass of oxygen atom(s) in each peptide. The relative intensities of hydroxylated versus non-hydroxylated peptide molecules in the P4H stably transfected S2 cells were substantially higher than those in the P4H non-transfected cells (FIG. 5, compare A with B), in agreement with the results from the amino acid composition analysis of overall minicollagen molecules.

Example 7 Triple-helical Conformation of Minicollagen XXI

Collagen triple-helices are resistant to pepsin degradation. To determine whether the COL1 domain of mC21 is in triple-helical conformation, purified mC21, with and without coexpression with P4H, were adjusted to pH 2.5 with 0.5 M of acetic acid, incubated at 22° C. and 37° C. for 15 min, followed by pepsin (25 μg/ml) digestion for 2 h to access helical stability. If mC21 is in a triple-helical conformation, pepsin digestion should lead to the removal of its NC1 domain and production of COL1 domain with the two cysteines present at the COL1/NC1 junction. As shown in FIG. 6, when mC21 was expressed in Drosophila alone, the entire interchain disulfide-bonded dimers and most monomers were disappeared after pepsin digestion at either temperature point (lanes 2 and 3), indicating that the proline residues in most of the mC21 molecules were under-hydroxylated and no triple-helical structure was formed. In contrast, some pepsin-resistant fragments derived from interchain disulfide-bonded trimers, (αCOL1)₃, and monomers (αCOL1) were remained following pepsin digestion of mC21 that had been coexpressed with P4H (lane 5). Moreover, these fragments were still existed when mC21 was pre-incubated at 37° C. (lane 6) followed by digestion with pepsin, suggesting that some of the mC21 molecules coexpressed in the P4H stably transfected Drosophila cells contained sufficient amounts of hydroxyprolines to stabilize the triple-helical structure. These pepsinization results demonstrated that coexpression of mC21 with P4H in Drosophila cells is capable of forming thermally stable triple-helical conformation.

The presence of pepsin-resistant monomeric fragments (αCOL1) after pepsin digestion (FIG. 6, lanes 5 and 6) indicated that these monomers may originally derived from the triple-helical, non-interchain disulfide-bonded trimers, which were dissociated into single chains under SDS-PAGE. To demonstrate that mC21, coexpressed in the P4H stably transfected Drosophila S2 cells, contained nun-interchain disulfide-bonded trimers, a gel filtration column was used to separate different mC21 species. Purified recombinant mC21, coexpressed in the P4H stably transfected Drosophila S2 cells, was separated by a Superdex 200 (HR 10/30) gel filtration column equilibrated with 0.1 M NaCl, 50 mM sodium acetate (pH 6.0), without prior denaturation (FIG. 7A, a), or after heating for 15 min in the presence of 5 mM DTT (FIG. 7A, b). Flow rate, 0.4 ml/min. The mC21, coexpressed with P4H, was eluted as two peaks under native conditions (FIG. 7A, a). The different peak fractions (numbered 1 to 3 in 7A) were analyzed by SDS-PAGE under non-reducing conditions. Gel was visualized by silver nitrate staining. The results showed that the early peak fraction (peak 1) contained a mixture of interchain disulfide-bonded trimers, dimers and non-interchain disulfide-bonded monomers, while the later peak fraction (peak 2) contained non-interchain disulfide-bonded monomers only (FIG. 7B, lanes 1 and 2).

The monomers present in the early peak fraction co-eluted with the interchain disulfide-bonded trimers indicated that these monomeric species were derived from non-interchain disulfide-bonded trimers. After heating in the presence of dithiothreitol, the three chains dissociated and eluted in the later peak fraction (FIG. 7A, b, peak 3) and only monomers were seen (FIG. 7B, lane 3). It is concluded that the mC21 triple helices, coexpressed with P4H in the Drosophila S2 system, contained a mixture of interchain and non-interchain disulfide-bonded trimers.

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. 

1. A recombinant insect cell, comprising a transfected genes encoding prolyl 4-hydroxylase consisting of the nucleotide sequences of SEQ ID NOs: 3 and
 5. 2. The recombinant insect cell as claimed in claim 1, wherein the cell is a Trichoplusia ni cell.
 3. The recombinant insect cell as claimed in claim 1, wherein the cell is a Drosophila melanogaster cell.
 4. The recombinant insect cell as claimed in claim 1, further comprising a transfected DNA fragment encoding a collagenous (COL1) domain and a C-terminal noncollagenous (NC1) domain of type XXI collagen.
 5. The recombinant insect cell as claimed in claim 4, wherein the transfected DNA fragment encoding a COL1 domain and a C-terminal NCI domain of type XXI collagen consists of the nucleotide sequence of SEQ ID NO:
 1. 